I heartily agree with IO540 on two points:
1) from professional experience and insight, with a little extension to Peter's phrase: first find the application software that does best what you want done. Then find the best O/S that it can run on, than find the best hardware the O/S can run on. "Best" having to concur with the limits of budget.
2) from hearsay, here and elsewhere, but VERY consistent - and also from limited personal experience: sunlight readability is the prime factor in choosing hardware. The Asus eePC 900 I tried is absolutely worthless under bright sun.